The Earth's climate is changing. The global mean temperatures are rising
(Fig. 1). The year 1995 is the warmest on record and the last 10 years (1987
-- 1996) are the warmest ten years on record. Glaciers are melting almost
everywhere and are disappearing in many parts of the tropics. At the same
time the composition of the atmosphere is changing, with clear evidence for
increases in carbon dioxide concentrations (Fig. 1). It is now well
established that these increases are due to human activities through the
burning of fossil fuels and deforestation. A central question then is
whether the climate changes and global increases in temperature are caused by
the human-induced effects. And do they matter? This article addresses the
issue of how the climate is changing and the anthropogenic effects, the rates
at which the changes are occurring, and goes on to discuss what this implies
for the future and the considerations in taking actions to address the
problem.

Fig. 1. Estimated changes in annual global mean temperatures and carbon
dioxide over the past 137 years relative to a 1961-90 base period.
Temperature data go through 1996 (red line), scale at left in ° C, and
the carbon dioxide values are from ice cores (blue dashed) and
for 1957 through 1995 from Mauna Loa, Hawaii (blue), scale at
right in parts per million by volume (ppmv) relative to a mean of 333.7 ppmv.

Evidence for climate change
The global mean temperatures have indeed risen over the past hundred years by
about 0.5° C (1° F) (Fig. 1). The temperature records have been assembled
from thousands of land and ocean observation sites covering a large,
representative portion of the Earth's surface and carefully controlled for
possible biases arising from station and instrument changes. Of
particular note in Fig. 1 is the relative warmth of the last 15 years or so.
Widespread melting of glaciers provides further evidence and, along with
thermal expansion of the oceans, has contributed to an increase in sea level
over the past century of about 15 cm (6 inches). Sea ice is melting back in
the Arctic and Antarctic regions. Northern Hemisphere snow cover has been
reliably monitored only since 1973 using satellite imagery and there is a
10% decrease in areal coverage since 1987 during the spring and summer
seasons.

Note from the figure that there is variability in temperatures from year to
year, and also from decade to decade superposed on the longer upward trend.
Presumably this variability is natural, for instance there is a mini global
increase in temperatures with El Niño, which is a natural warming of the
tropical Pacific Ocean that occurs every 3 to 6 years or so. The range of
natural variability in global temperature seems to be about ±0.2° C, so
that it is only following the late 1970s that global mean temperatures can be
seen emerging from the noise of natural variability. There was a substantial
warming in the Arctic and North Atlantic from about 1910 to 1940 which is
reflected in the global mean, and which is likely to have had origins related
to the ocean and atmospheric circulation. A consequence is that warming in
that region since then has been small, but nor has it cooled much. Some
cooling has taken place in the North Atlantic and central North Pacific and
is known to be a consequence of changes in the atmospheric circulation which
naturally creates southerlies in some regions and northerlies in other
regions, so that spatial patterns of temperature change are not and should
not be expected to be uniform.

Other climate changes have also occurred in humidity, precipitation (both
average values and intensity), storms and other phenomena, but these are
discussed further later as their relevance becomes apparent.

Human influences

By modifying the Earth's environment in various ways, human activities are
changing the climate, although it is difficult to ascribe the effects with
certainty. The burning of fossil fuels pollutes the atmosphere and alters
the balance of radiation on Earth through both visible particulate pollution
(called aerosols) and gases that change the composition of the atmosphere.
The latter are referred to as greenhouse gases because they are relatively
transparent to incoming solar radiation, while they absorb and reemit
outgoing infrared radiation, thus creating a blanketing effect which results
in warming. For example, carbon dioxide concentrations in the global
atmosphere have increased by about 30% from human activities over
preindustrial values (Fig. 1). Emissions of CO² into the atmosphere continue
to grow and concentrations increase because CO² has a long lifetime in the
atmosphere. Several other greenhouse gases (methane, nitrous oxide,
chlorofluorocarbons) are also increasing from human activities (mostly
agriculture, land use changes and industry). Global warming and associated
climate change is expected as a result. Increases in atmospheric aerosols
may offset global warming by blocking the sun's radiation and by increasing
the brightness of clouds which also reflect radiation back to space. These
effects are mainly localized because the lifetime of aerosols is only a week
or so as they are washed out of the atmosphere by rain.

Increases in greenhouse gases in the atmosphere produce global warming
through an increase in heating at the surface, and thus not only increase
surface temperatures but, as most of the heating at the surface goes into
evaporating surface moisture, it also enhances the hydrological cycle.
Increases in moisture content (i.e. the humidity) of the lower atmosphere
(which is observed in many places) is a consequence and this moisture
provides a resource for all precipitating weather systems, whether they be
thunderstorms or extratropical rain or snow storms, because all of these
systems feed upon the available moisture within their reach. This means
enhanced rainfall or snowfall events, thus increasing risk of flooding, which
is a pattern observed to be happening in many parts of the world. In
particular for the United States, atmospheric moisture content is observed to
have trended upwards by about 10% over the past 20 years or so and
observations clearly show that heavy rainfall and snowfall events are
increasing at the expense of more moderate falls. While a few percent
increase may not seem like much, it can be the straw that breaks the camel's
back --- or just what is needed to broach a dyke, as was the case in Grand
Forks, North Dakota, in April 1997, when melting snow caused extensive
flooding in the Red River basin (the river peaked at 54 feet above flood
stage and the levies and dyke system broke at 51.5 feet above flood stage.)
Increased evaporation also leads to the expectation of enhanced droughts
(earlier onset, longer lasting, greater intensity) and greater wilting of
vegetation. It also means that average precipitation should increase as a
whole, and this is observed mainly over land in mid to high latitudes.

Other climate phenomena are exhibiting very unusual behavior, for example El
Niño which disrupts weather patterns around the globe causing floods and
droughts. However, since the late 1970s there are clear signs that El Niño
is becoming more frequent compared with the previous hundred years of record
and the associated changes in precipitation in the tropics dominate and
complicate the record there. A new major El Niño is currently underway and
should continue to develop throughout this year. Is this behavior change
related to global warming? It could be, but we cannot yet be sure.
Nevertheless, because El Niño brings droughts to Australia, Indonesia,
parts of southern Africa, Southeast Asia, northeast Brazil and Columbia, and
floods to the west coast of South America and some other places, these
naturally occurring floods and droughts are apt to be worse with global
warming effects superposed.

Causes of change

It is one thing to identify changes in climate that are unexpected, based on
previous observed behavior. These changes certainly indicate that something
is going on. But it is much more difficult to definitively say the changes
are caused by the human-induced effects. A parallel here is trying to link
lung cancer to smoking. There are always some people who smoke that do not
get lung cancer, and some who get lung cancer who do not smoke. Yet the
evidence is compelling that there is a link.

After carefully examing all the available evidence, the Second Assessment
Report in 1995 of the Intergovernmental Panel of Climate Change (IPCC) has
concluded that the balance of evidence suggests a discernible human
influence on global climate. The IPCC is sponsored by the World
Meteorological Organization and the United Nations Environment Program and
the 1995 assessment involved over 2000 scientists from all over the world.
The evidence examined included all the observations of changes, including
paleoclimatic indicators from the distant past, and the patterns of changes,
such as how temperatures are changing with altitude and geographically.
Climate models (see below) also played a role by estimating what changes
should have occurred given observed changes in atmospheric composition, the
sun and other effects (such as from volcanoes) over the past century, and by
helping to assess the levels of natural variability. Thus far, the
human-induced effects are relatively small compared with the huge day-to-day
variations of weather. In addition, natural variability occurs on
seasonal-to-interannual and decadal timescales and contributes to the climate
record for the planet Earth which makes any anthropogenic signal in the
climate record hard to notice.

Climate models and prediction

In environmental science, it is not possible to carry out experiments in the
laboratory, as happens in physics and chemistry. Imagine if we could create
two planet Earth's identical to our own in every respect, and then observe
how the climates and the consequences for the society of each planet evolved
as different actions were taken. For example, on one planet the people might
decide to continue headlong on our current course, dumping huge amounts af
carbon dioxide and visible particulate pollution into the atmosphere. While
on the other, the people might take strong actions to limit emissions. We
cannot do this with a physical model. Therefore we have to try to understand
the climate system well enough to build a good model of the climate system in
a computer, and use this model to perform the experiments. Climate models
are based upon physical laws represented by mathematical equations and are
solved using numerical methods on computers. They encapsulate our current
understanding of the climate system and the physical processes involved.
They integrate all the knowledge we have from observations and theory, and
they have been extensively tested and evaluated using observations. Hence
they can be used as a tool in climate research. Of
course all models are wrong because, by design, they depict a simplified view
of the system being modeled. But many models are nevertheless very useful!

Model results are judged by considering all the assumptions and
approximations, and it is generally inappropriate to take the model result at
face value. Probably the single greatest uncertainty in climate models stems
from their treatment of clouds, and handling the enormous variety and
variability of clouds poses a special challenge. While uncertainty exists in
the climate models, the complexity of the climate system is such that models
often provide the only means of quantifying the result of a perturbation to
the climate system. Accordingly, computer climate models are used to make
projections of how the climate may change in the future in response to
further changes in atmospheric composition.

What is done is that first a control climate simulation is run with the
model. Then the climate change experiment simulation is run, for example
with increased carbon dioxide in the model atmosphere. Finally the
difference is taken to provide an estimate of the change in climate. There
is considerable uncertainty in what the emissions into the atmosphere will be
of both carbon dioxide and aerosols (and other gases), and so a number of
possible scenarios, which depend on whether we collectively act to limit
emissions, are used in the experiments. These emissions are translated into
expected concentrations of gases, and it is clear that carbon dioxide
concentrations will continue to increase unless emissions are substantially
reduced below today's values. It is estimated, for instance, that carbon
dioxide concentrations will likely increase to 700 parts per million by
volume (ppmv) by the year 2100 (compared with 360 ppmv in 1996 and 280 ppmv
200 years ago), see Fig. 1. Then the best estimates are that global mean
temperatures will continue to increase, by about 1.0 to 3.5° C (2 to 6°
F) by the year 2100 and sea level will increase by another 15 to 95 cm (6 to
37 inches). Nevertheless, because they undergo the biggest percentage
change, extremes are the main way we will notice climate change: the very hot
and/or humid days, the heavy rains, the droughts, the fewer very cold days,
and so on.

Because none of the scenarios are really realistic, and many other effects,
such as changes in land use by humans, are not included, the projections are
not forecasts and should not be treated as such. Used appropriately,
however, they provide useful information for planning and as a basis for the
needed public debate on what actions should be implemented.

Does climate change matter?

So what does all this mean? What, if anything should be done? Why should we
care? Clearly addressing these questions involves much more than scientific
judgements but relates to value systems and considerations such as to what
kind of stewards we are for the planet Earth and what kind of environment we
leave to the future generations.

There has been a politicization of environmental science which Congressman
George Brown has written about in the March 1997 issue of Environment.
Somehow what we can say about the science becomes mixed up with advocacy on
what we should do about the conclusions. It should be possible to separate
these two things. The first step is to make the best scientific assessment
as to what can be said about the problem in question, including all the
caveats and uncertainties, and then the public and politicians may debate and
decide what actions to take while accounting for all world views. It can be
argued that it is impossible for a scientist not to be biased and to
therefore put a particular slant on his or her results. But this is where
the process of building a consensus plays a key role. In the IPCC scientific
assessment, there were scientists from all parts of the political spectrum
represented. Yet the vast majority were focussed only on making the best
statements possible about the science, given our current understanding, in a
very open process.

I have found it helpful to recognize that there are several world views that
help to characterize the issues. At perhaps one extreme is the
environmentalist who believes that we should conserve the environment and who
therefore has a political agenda that calls for actions to mitigate and abate
the increases in greenhouse gases, for instance with policies or incentives
designed to limit emissions into the atmosphere. At another extreme are
those who think that change is inevitable, but technology will solve all
problems and therefore that we can just adapt to climate change as it
happens. Of course most people fall somewhere in between.
For example, one growing approach is to recognize limits to growth
and subscribe to sustainable development which places a premium on use of
renewable resources.

In addition, another class of people are those who have vested interests in
the current situation. Their strategy is often to denigrate the issue or
deny that there is an issue at all. Like the tobacco companies who have long
denied the addictive effects of nicotine and adverse effects of smoking on
lung cancer, countries rich in oil and fossil fuel companies spend huge
amounts of money to publish often misleading or invalid material to deny that
there is a problem. It is noteworthy that the only two countries who
obstructed progress and continually objected to the IPCC working group 1
report in the intergovernmental plenary in Madrid, Spain in November 1995
were Saudi Arabia and Kuwait. Oil companies, such as Exxon, publish selective
and biased views in their newsletter to shareholders. Western Fuels, a
cooperative which provides coal to generate electricity, wages negative
advertising campaigns and funds the work of skeptics. A typical strategy,
for instance, has been to focus on a short satellite record of temperatures
in the lower part of the atmosphere which shows a downward trend since 1979.
However, because this record is made up of segments from 8 different
satellites, it has been shown that the downward trend is spurious and arises
from how the segments are joined up, a point ignored by the skeptics. This
argument also conveniently ignores another more reliable satellite record
that shows rising global temperatures for the same period. It further
ignores the much longer surface record of rising temperatures (Fig. 1). This
kind of selective use of information is designed to mislead and widen the
uncertainties that already exist, leading to inaction. Moreover, the skeptics
often exaggerate the problem thereby suggesting that the changes needed would
be very disruptive to the economy, again discouraging the taking of any
action.

The neglect of information does not happen when a consensus is built such as
that for the IPCC. Instead all information is evaluated (including that from
skeptics) and taken into account. Consensus science may not produce the best
and latest result, but judgements are made as to which results are truly
established.

What to do?

It is important to first realize that this human experiment we are performing
on planet Earth is underway and cannot be turned off if we do not like the
way it is going or the eventual outcome because of long lifetimes of carbon
dioxide (centuries) and other greenhouse gases in the atmosphere and because
of the thermal inertia of the oceans. The oceans overturn very slowly and
take hundreds of years to fully adjust to changes occurring, so that
manifestations of changes that have already occurred are not fully seen.

It is clear that at present effects of global warming are fairly small, but
they are unmistakably emerging and having impacts. The insidious thing about
global warming is that the changes are always in one direction, and thus they
accumulate with time. Moreover the changes will continue long into the
future even if we want them to stop and even in the unlikely event that we
abruptly reduce carbon dioxide emissions. While some climate changes, such
as longer growing seasons, may be beneficial for some activities, the
climate changes will not stop. Other projected changes, such as rising sea
levels, are more clearly likely to have only adverse effects. In fact,
however, it is the process of change itself that is very disruptive. It is
disruptive to the natural environment and ecological systems which have not
experienced rates of change as large as those projected in the past 10,000
years. It is also disruptive to human systems, agriculture, water resources,
fisheries, energy use, and so on, because suddenly we find that the recent
past weather is no longer a useful guide as to what to expect. For instance,
if the return period of a particular severe storm changes from once per
hundred years to once in fifty years, design criteria for dams, levies,
buildings and so on become obsolete. Thus change disrupts planning. Our
understanding of the climate system is such that it is likely for there to be
unanticipated surprises which produce very disruptive impacts at least in some
areas.

Whether these arguments are compelling to the reader or not probably depends
somewhat on their view of the world, their place in it, and the extent to
which they care about the world we leave the next generation. My view, all
things considered, is that the case is very strong that at least we should
take actions to slow the process of change down, and this means slowing the
rate of increase of greenhouse gases in the atmosphere considerably.

Dr. Kevin E. Trenberth is Head of the Climate Analysis Section
at the National Center for Atmospheric Research, which is sponsored by the
National Science Foundation. He was a convening lead author of the 1995 IPCC
Scientific Assessment of Climate Change and he is Co-chair of the Scientific
Steering Group for the World Climate Research Programme's Climate Variability
and Predictability (CLIVAR) program. He is a fellow of the American
Meteorological Society and American Association for Advancement of Science,
and an honary fellow of the Royal Society of New Zealand.